Two-dimentional (2D) Ti₃ C₂ T_x MXene has attracted significant attention in non-lithium-ion batteries due to its excellent electrical conductivity, high volumetric capacity, and ability to accommodate intercalants. Rechargeable magnesium batteries with Mg metal anodes are noted for their high theoretical energy density, potential safety, earth abundance, dendrite-free Mg²⁺ plating/stripping mechanism on the anode side, and low cost. Nevertheless, owing to the large polarity of divalent Mg²⁺ ions, the insertion of Mg²⁺ into the MXene layers suffers from sluggish kinetics, limiting the performance for storage of Mg²⁺ ions. Herein, a simple self-assembly strategy is demonstrated to achieve high magnesium ion storage capability with pillar-structured Ti₃ C₂ T_x MXene by intercalating a hyperbranched polyethylene ionomer containing quaternary ammonium ions. The ionomer intercalation/modification leads to the expansion of interlayer spacing of the MXene and, meanwhile, improves its affinity to low-polarity THF-based electrolyte. The delaminated ionomer-modified MXene shows significantly improved electrochemical performance as a cathode material for Mg batteries. It shows a promising cycling stability with a capacity retention of 86% after 400 cycles at 200 mA g^-1 , as well as outstanding high-rate performance with a capacity of 110 mAh g^-1 retained at 1,000 mA g^-1 relative to 213 mAh g^-1 at 20 mA g^-1 .